Various patents and publications describe substrate processing apparatuses that use sonic energy and fluids for processing substrates.
U.S. Pat. No. 5,456,759 describes a substrate processing apparatus that uses transducers placed directly inside of a processing chamber to produce sonic energy. The processing chamber includes a liquid gas.
However, the substrate processing apparatus described in U.S. Pat. No. 5,456,759 has some disadvantages. Piezoelectric transducers have bad corrosion resistance and deteriorate quickly when they come into contact with liquid gases. Corroding transducers can produce contaminants, which can undesirably deposit on the substrates being processed.
U.S. Pat. No. 6,228,563 discloses a substrate processing apparatus that uses a sonotrode to generate acoustic waves. A sonotrode is a rod, which is placed through the wall of a processing chamber of a substrate processing apparatus. Outside of the processing chamber, piezoelectric transducers transmit their vibrations to the rod, which transmits energy to a fluid inside of the processing chamber.
The system described U.S. Pat. No. 6,228,563 has some disadvantages. First, it is difficult to obtain a high pressure sealing between the processing chamber and the sonotrode without absorbing most of the sonic energy. The sonotrode's vibrations dangerously decrease the life-time of this sealing system. Second, the sonotrode emits vibrations in all directions. It is difficult to control the sonic waves produced by the sonotrode, particularly at its end. It has been demonstrated that the waves are the strongest at the sonotrode's end and these waves can damage, for example, a pattern on a semiconductor wafer.
U.S. Pat. No. 5,522,938 describes a substrate processing apparatus where megasonic acoustic waves are generated using an antenna that is oriented perpendicular to the orientation of the substrate. In this way, the generated acoustic waves travel along the wafer resulting in a streaming effect.
The system described in U.S. Pat. No. 5,522,938 has some disadvantages. First, it is difficult to produce waves of the type mentioned in U.S. Pat. No. 5,522,938 uniformly onto the width of the substrate. It is also difficult to do so in a cylindrical processing chamber. The cylindrical geometry of the processing chamber is dictated by the high processing pressures inside of the processing chamber. Second, in U.S. Pat. No. 5,522,938, the pressure that is used is at the critical value so that the fluid can oscillate between gas phase and the supercritical state. This is not very efficient. There is no real phase change between the gas phase and supercritical state of a fluid, because variations in the fluid's characteristics (e.g., density, surface tension, etc.) are very small if the pressure variation is also small.
Accordingly, improved substrate processing apparatuses and methods using dense phase gases and sonic energy are desirable. Embodiments of the invention address the above problems and other problems, individually and collectively.